In a gas turbine engine, it is well known that air is pressurized in a compressor and used to combust a fuel in a combustor to generate a flow of hot combustion gases, whereupon such gases flow downstream through one or more turbines so that energy can be extracted therefrom. In accordance with such a turbine, generally, rows of circumferentially spaced turbine rotor blades extend radially outwardly from a supporting rotor disk. Each blade typically includes a dovetail that permits assembly and disassembly of the blade in a corresponding dovetail slot in the rotor disk, as well as an airfoil that extends radially outwardly from the dovetail.
The airfoil has a generally concave pressure side wall and generally convex suction side wall extending axially between corresponding leading and trailing edges and radially between a root and a tip. It will be understood that the blade tip is spaced closely to a radially outer turbine shroud for minimizing leakage therebetween of the combustion gases flowing downstream between the turbine rotor blades. Maximum efficiency of the engine is obtained by minimizing the tip clearance or gap such that leakage is prevented. However, this strategy is limited somewhat by the different thermal and mechanical expansion and contraction rates between the turbine rotor blades and the turbine shroud, and the motivation to avoid an undesirable scenario of having excessive tip rub against the shroud during operation.
It will be appreciated that conventional blade tips include several different geometries and configurations that are meant to prevent leakage and increase cooling effectiveness. One approach, referred to as a “squealer tip” arrangement, provides a radially extending tip rail that may rub against the tip shroud. The rail reduces leakage and therefore increases the efficiency of turbine engines. However, the tip rail of the squealer tip is subjected to a high heat load and is difficult to protect from wear—it is frequently one of the hottest regions in the blade. Accordingly, blade tip rails are subjected to hot gases and wear, which can cause high stresses and thus require periodic repair.
To protect turbine components, various exterior surface coatings are typically applied over the base materials thereof. Exterior surface coatings refer to coatings such as but not limited to anti-oxidation coatings like an overlay coating and/or a bond coating. A thermal barrier coating (TBC) may also be disposed over a bond coating. During surface life, cracks can form in the exterior surface coating(s) and can damage the exterior surface coating(s) and/or the base material of the turbine component. The exterior surface coating can also be oxidized and/or damaged due to wear. The damage may weaken the turbine component and/or alter the shape of the turbine component.
Repairing gas turbine components can be costly and time consuming. For example, many repairs require removal of at least some of the exterior surface coatings and large, time consuming material removal and replacement, e.g., using additively manufactured coupons. Where material is replaced, the process oftentimes requires exposure of the turbine component to welding and a high temperature heat treatment, each of which can damage the single crystal material of the component. The repair itself can also oxidize the exterior surface coatings and/or other parts of the turbine component that are not removed. This situation also impacts repair of a blade tip rail. Tip rail repair has the additional challenge that there may not be enough remaining tip rail thickness and adequate attachment points to create a long-lasting repair. No solution is currently available to adequately repair a turbine component or a blade tip rail thereof where the repair is made of suitable material, the process avoids a heat treatment at high temperature during the repair, and/or the repair can address the presence of an exterior surface coating on the component. Current processes also do not exist for manufacturing a blade tip rail without facing the above challenges.